Electrolytic refining of aluminum



Nov. 24, 1931. .1. WEBER ET AL ELECTROLYTIC REFINING OF ALUMINUM FiledJan. 8, 1929 CaT/mde a Patented Nov. 24, 1931 UNITED STATES PATENTOFFICE JULIUS WEBER AND ALFRED V011 ZEEIBLEDER, OI NEUHAUSEN,SWITZERLAND, A3-

SIGNORS TO ALUMINIUM INDUSTRIE AKTIENGESELLSCEAF'I', OI NEUHAUSEN,

SWITZERLAND, A JOINT-STOCK COMI'ANY OF SWITZERLAND ELECTROLYTIC REFININGOI ALUMINUM Application filed January 8, 1929, Serial No.

This invention relates to the electrolytic refining of aluminum, andincludes a novel process for producing highly pure metallic aluminumfrom less pure metal, and a spe- .6 cial electrode used therein. Theinvention is directed particularly to the extraction of refined aluminumin a solid and dense form from pig or ingot aluminum, aluminum alloysand the like. It constitutes an improvement more particularly in thatrefining processes wherein the electrodes, used in a solid form, areimmersed in a molten electrolyte, for example containing halogen salts,which electrolyte has a melting point lower than that of eitherelectrode and is maintained in a molten condition below the meltingpoint of either electrode. Such a process has been described in and madethe subject of the prior application of Weber & Hauser, Serial N 0.113,561, filed June 3, 1926, patented April 16, 1929, No. 1,709,759,also in an improved form in the application of Weber & Zeerleder, SerialNo. 195,861, filed June 1, 1927. These prior applications may bereferred to for details and features not herein fully described.

The general objects ofthe present invention are to render more efiicientthe refining process, to increase the purity of the product,

30 and to enlarge the rate of output or production. Other objects andadvantages of the invention will be explained in the hereinafterfollowing description of an illustrative embodiment thereof or will beunderstood to those conversant with the subject. To the attainment ofsuch objects and advantages the present invention consists in the novelelectrolytic process of producing pure metallic aluminum, and the novelmaterials or electrodes used therein, and the novel features of process,operation and means herein described or illustrated.

The nature of the present improvement can best be explained by firstpointing out the conditions which pertain to the processes already knownand the considerations involved in further perfecting or im roving thesame. In practicing thedescribe refining process we have determined thatthe aluminum which is deposited in solid form at the cathode may solvedto a depth of several millimeters,

380,987, and in Germany February 4, 1928.

be rendered the more pure by maintaining the current density lower atthe anode. The reason underlying this is that the selective action ofthe electric current on the various constituents of the anode is farmore effective at low than at high current densities. By the use of lowcurrent densities it is possible particularly to restrain the taking upof iron at the anode, although iron, which is invariably present in pigaluminum, has a considerably greater tendency to enter into solutionthan other constituents, for example silicon, which is an equallyunavoidable and undesirable impurity in pig aluminum.

While, as stated, the use of lower current density at the anodes isfound to enhance the purity, on the other hand We find that far ighercurrent densities are permissible at the cathodes without impairing thepurity of the deposited metal.

Observation of the described process shows that the electrolytic tensionof the bath commences to increase at a rapid rate as soon as thealuminum at the anode has been disthe remaining undissolvedconstituents, consisting chiefly of iron and silicon, forming a sort ofcoating or crust. This coating of impurities upon the anode acts as amembrane or skin tending to resist the passage of the electric currentand thereby increasing the electrolytic tension of the cell. Moreoverthis resulting increase in tension tends to bring about still otherobstacles to the action taking place at the anode, due partly to localelevations of temperature, and partly to a tendency to increase theelectrolytic potential at the surface of the anode. The harm of thesedisadvantageous actions is that in due time the increasing potentialwill reach that potential at which the iron impurity starts to dissolveat the external layers of the anode.

\Ve have determined however that the described interfering alteration ofelectrolytic tension at the surface layers formed by the undissolvedimpurities of the anode is materially smaller at the low currentdensities than at high ones. In other words there is a criticalpotential at which the iron and other undesirable impurities commence togo 100 I anode,

on the time,

9 sions (length and breadth) into the solution, which critical point isnot reached when usin the low current densities, or is only reach whenthe aluminum has been dissolved from the deeper layers of the whereaswith the hig er current densities a reciable amounts of iron and siliconinevita y pass into solution.

While it therefore appears that'the lower current densities aredesirable in order to 10 reduce the amount of impurity in the productdeposited at the cathode, yet, on the other hand, it is considered to benecessary, in a commercial sense, to operate with current densities ashigh as possible in order to carry rocess in t e most economical way toafior the maximum output. The reason for this latter consideration isthat the hi h current densities produce at the oath es moremetal perunit of surfacein a given thereby not only increasing rate of output,but effecting an economy in electrolyte material and a saving space andcost. For instance, production economies may be effected by reducing thesize of the electrodes and thereb the quantity of electrolyte, which atthe same time means a substantial saving of current employed in theelectric heating of the electrol te.

Clearly t erefore it would not be economical to employ a relativelysmall size of cathode in conjunction with a large size of anode inorderto permit high current densities at the cathodes and low densities atthe anodes, because by this plan it would not be possible to reduce thesize of the cell, nor the quantity of electrol te nor the amount ofcurrent required. either are the desirable results obtainable-by the useof cathodes and .anodes which have the same size or dimenand the usualplatelike form with plane sides, because with this plan it would not bepossible simultaneously to maintain high current densities at thecathodes and low densities at the anodes. In order to insure a thoroughexhaustion of the anode material, anodes in the form of a plain platewould have to be made relatively thin, and would therefore have to bereplaced with objectionable frequency, or

9 else the total amount of electric current assed through the bath wouldhave to be resuced considerably, consequently rendering far less therate of metal produced per unit of surface, resulting in a mostuneconomical system.

According to the present invention the difliculties referred to arealtogether avoided and the desired advanta es are obtained by theemployment, in the fescribed process, of anodes each having asubstantially larger surface than an anode of the same size ordimensions and the same weight, but with plain l or flat sides orsurfaces. The present invention is characterized by presenting the solidanode metal in a form so departing from a creased, not merel It is notat first sight clear that such anodes,

formed with elevations and depressions sub-' stantially enlarging thesurface of exposure, can be used in the described process, because ofthe known fact that under ordinary conditions the lines of current flowprefer prominent surface portions or elevations to flat portions orderessions and therefore tend to be far more ense at angles, jections thanat flat surfaces, depressions or ooves. From this consideration it mighte expected that the disintegration of the special anodes contemplated bythis invention would tend to take place only at the outedges andprostanding portions or elevations and not at the flat portions ordepressions, which, if true, would render practically impossible asystematic or complete exhaustion of the anode material.

We have determined however by test and demonstration that with thepresent invention this drawback does not occur, and that,notwithstanding the expected tendency, the distribution of the currentremains practically uniform over all of the surface of the anode. Weattribute this largely to the character of electrolyte employed, whichhas very good conductivity. Actually therefore, the special anodes ofthis invention, with their large exposed surface, are decomposed in asubstantially homogeneous manner. The invention thus makes ossible a farbetter utilization of the anod material than is obtainable with the useof anodes havingthe same size or dimensions and weight, but having lainside surfaces thlc 'er in section.

Anodes, according to this invention, may be of various form or design,and may afford a surface from twice to eight times the extent of ananode havin the same length and breadth but with flat sides. Althou h inusing a simple plate or slab as an ano e the tension of the electriccurrent tends to exceed the admissible limit or critical point describedas soon as thirty per cent more or less of the contained aluminum hasbeen dissolved, we find that an anode of the same length. breadth andweight, but whose surface has beenmade. I large according to the presentinvention, may

and being accordingly 113,561 and containing electrodes and electrolyteoperating according to the present invention.

The cell 5 is shown as having an entering pipe 6 to permit thecirculation or replacement of electrolyte, and an exit pipe 7. Theelectrolyte or bath 8 is a molten bath having a melting point lower thanand maintained below the melting point of either electrode. As describedin the prior applications such bath may contain halogen-salts, namelyhalides of aluminum and halides of another metal or metals havingalkaline properties,

and the bath having good electric conductivity. A typical bath compriseshalogen salts of aluminum together with halogen salts either of one ormore alkali metals or one or more alkali earth metals, or all of thesesalts,

as more particularly disclosed in the prior applications. A simple bathconsists of NaCLAlCl- In the bath are shown immersed alternated catholes9 in the form of plain slabs or plates and anodes 10 of s ecial form,but of the same size, that is lengt and breadth, as the cathodes.

The special anode 10 is shown as formed to present enlarged exposedsurface by means of a system of elevations or ribs 11 and de pressionsor grooves 12. This undulated form materially increases the surface.Preferably the ribs and rooves are formed at both sides of the an e andin such manner that the ribs at one side are 0 posite to the grooves atthe other side, so that the anode is given a substantially uniformthickness from surface to surface at every oint, and presents no partswhich, being thlcker than others, would still contain aluminum, whilethe thinner parts of the anode would already be completely exhausted. Ananalogous increase of exposed surface may be effected in various otherways, for example by having the ribs and grooves run in intersectingdirections so as to form prominences of the shape of pyramids. The anodemay be produced not only by casting but b rolling or by extrusion intoappropriate s apes.

The electrical connections may be made in various ways, but for example,as shown, the

several anode plates 10 and the cathode plates 9 are connected inparallel. Thus a conductor 13 connects the cathodes and a conductor 14connects the anodes, and the roper current may be supplied to these conuctors both for the purpose of heatin the electrolyte to maintain it inmolten con ition, and for electrode osition. Or the heating may beeffecte by a secondary current or by external means. Other ways ofelectrical connections may be applied, for example the connection inseries, es ecially according to the well known Hay en or series system,which has been known and used for many years, for example in copperrefining, and which is described in. various publications, such asEnineering and Mining Journal, New York,

01. LIV (1892) page 126. p

. An illustrative instance, within the principles described, the anodesand cathodes may be of such relative character or arranged that a cellvoltage of 0.3 volt between anodes and cathodes the current density atthe anodes may be 0.5 amperes or less per square decimeter, and at thecathodes 1.0 amperes or more, with an appropriate distance of separation and electrolyte of'high conductivity as described.

There has thus been described a process for producing highly puremetallic aluminum from less pure metal and a'special electrode or anodefor use therein'embodying the. principles and attaining the objects ofthe present invention. Since various matters of process, ingredient andapparatusmay be variously modified without departing from the principlesof the invention, it is not intended. to limit the invention to suchmatters except so far as set forth" in the appended claims.

What is claimed is:

1. A process of electrolytic extraction of pure aluminum by therefiningof pig alumi num, aluminum alloys or the like, and wherein both thealuminum containing anodes made of the material to be refined and thecathodes are used in a solid form together with an electrolytic bathhaving a melting point lower than that of either electrode, andconsisting of halogen salts of aluminum and of alkaline and/or alkalineearth metals, characterized by the use of an anode having asubstantially larger surface than an anode of the same size and the sameweight but with plane surfaces.

2. The electrolytic process of producing highly pure metallic aluminumfrom less pure metal wherein the electrodes in solid form are immersedin a molten electrolyte containing halogen salts and having a meltingpoint lower than and maintained below the melting point of eitherelectrode and characterized by presenting the solid anode metal in a.form substantially departing from a plain slab or plate so as to affordsubstantially enlarged surface of exposure, as compared with that of thecathode.

3. The electrolytic process of producing highly pure metallic aluminumfrom less pure metal wherein is used a molten electrolyte having amelting point lower than that of the anode, and characterized by the useof a solid anode in a form departing substan- 5 tially from a plain slabor p ate so as to present extensive surface of exposure.

4. The electrolytic process of producing highly pure metallic aluminumfrom less pure metal wherein the electrodes in solid form are immersedin a molten electrolyte possessing good conductivity and having amelting point lower than and maintained below the melting points of theelectrodes, and characterized by presenting the impure metal as solidanodes having substantial elevations and depressions presenting extendedsurface exposure.

5. The combination with a molten electrolytic bath and means forsupplying current,

90 a series of cathode plates and alternated therewith a series of solidanodes formed with substantial surface elevations and depressions, so asto present a larger surface than that of the cathode, whereby to causealarger current density at the cathode than at the anode.

6. The electrolytic process of producing highly pure metallic aluminumfrom less pure metal wherein the electrodes in solid form are immersedin a molten electrol te containing halogen salts and havingameltmg pointlower than and maintained below the melting point of either electrodeand where in the anodes and cathodes are of such difi'erent character orarrangement that the current density at the anodes isv substantiallyless than at the cathodes.

In testimony whereof, this specification has been duly signed by:

JULIUS WEBER.

ALFRED VON ZEERLEDER.

